Previous research has shown that endogenous digitalis-like factors (EDLF) are increased in humans and laboratory animals with clinical and experimental hypertension. However, the extent to which EDLF increases with age is not known. The chemical structures of various EDLF remain to be clarified, but human plasma and urine appears to contain both a ouabain-like substance and a digoxin-like substance which may be a bufadienolide. The Baltimore Longitudinal Study on Aging provides the opportunity to determine whether the plasma and/or urinary ouabain-like and bufagenin-like factors are associated with age.